The semiconductor materials with wide bandgaps such as diamond, silicon carbide, and gallium nitride have become of significant interest in recent years because their wide bandgap characteristics provide them with the capability to emit light of higher energy (with correspondingly higher frequency and shorter wavelength) than do other semiconductor materials such as silicon or gallium arsenide. In particular, silicon carbide, gallium nitride, and certain other Group III nitrides have bandgaps large enough to produce visible light throughout the visible spectrum, including the higher-energy blue portion. They thus provide the basis for semiconductor lasers and light emitting diodes (LEDs) with blue and green emissions.
Of these materials, gallium nitride is of particular interest because it is a direct semiconductor, i.e., the transition from the valence band to the conduction band does not require a change in crystal momentum for the electron. As a result, the transition produces light very efficiently. In contrast silicon carbide is an indirect semiconductor; the bandgap transition energy is given off partly as a photon and partly as vibrational energy. Thus, gallium nitride offers the advantage that for a given operating voltage and current, it will produce light more efficiently than silicon carbide
As with other semiconductor materials, however, the first step in forming a workable photonic devise is to build-up or otherwise obtain a suitable crystal structure with the desired active layer. Because of the differences in the structural characteristics of semiconductor materials, however, particularly their crystal lattice structures, the materials which will workably support Group III nitride active layer devices are somewhat limited.
Presently, commercially available structures for a light emitting diode photonic device are formed of a gallium nitride or related Group III nitride active layer on a sapphire substrate. Sapphire (Al.sub.2 O.sub.3) provides a relatively close lattice match to Group III nitrides, but also suffers certain disadvantages, the most limiting of which is its electrically insulating character. Thus, when Group III nitride active and buffer layers (i.e., the intermediate layers that provide a crystal structure transition from the substrate to the active layer) are built on sapphire, the sapphire cannot be used as a conductive pathway to the active portions of the device. This limits the type of devices that can be designed and produced on sapphire, and in particular limits the ability to form "vertical" devices in which the device contacts can be placed on opposite surfaces of the device with a direct conductive path through the substrate, buffers, and active layer, and the other contacts on the opposite of the device.
Accordingly, interest, including that of the assignee of the present invention, has focused upon the use of other materials as substrate candidates for Group III nitride photonic devices. Silicon carbide (SiC) is a particularly attractive candidate because it can be made conductive, has a lattice match that can be appropriately buffered to a Group III nitride active layer, and has excellent thermal and mechanical stability.
Nevertheless, silicon carbide's crystal lattice structure is such that some of the best candidates for an appropriate Group III buffer layer on a silicon carbide substrate are insulating rather than conductive. Thus, although the silicon carbide substrate can be made conductive, some of the preferred buffer layers between silicon carbide substrates and Group III active layer photonic devices remain insulating, thus minimizing the advantages of the conductive silicon carbide substrate.
For example, aluminum nitride (AlN) provides an excellent buffer between a silicon carbide substrate and a Group III active layer, particularly a gallium nitride active layer. Aluminum nitride is, however, insulating rather than conductive. Thus, structures with aluminum nitride buffer layers require shorting contacts that bypass the aluminum nitride buffer to electrically link the conductive silicon carbide substrate to the Group III nitride active layer. As noted above such shorting contacts preclude some of the more advantageous device designs.
Alternatively, conductive buffer layer materials such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), or combinations of gallium nitride and aluminum gallium nitride can eliminate the shorting contacts. In turn, eliminating the shorting contact reduces the epitaxial layer thickness, decreases the number of fabrication steps required to produce devices, reduces the overall chip size, and increases the device efficiency. Accordingly, Group III nitride devices can be produced at lower cost with a higher performance.
Nevertheless, although these conductive buffer materials offer these advantages, their crystal lattice match with silicon carbide is less satisfactory than is that of aluminum nitride. Accordingly, when epitaxial buffer layers of gallium nitride, aluminum gallium nitride, or combinations thereof are grown on silicon carbide substrates, they tend to produce excessive cracking in subsequent epilayers that are required for photonic devices such as light-emitting diodes or laser diodes.
Thus, there exists a need for a buffer structure that offers the crystal lattice match advantages of aluminum nitride and yet which at the same time offers the conductivity advantages of gallium nitride or aluminum gallium nitride and that can be used in conjunction with conductive silicon carbide substrates rather than insulating sapphire substrates.